The isolation of antibodies directly from immune donors offers the advantage of fully exploiting the strength of the human antibody response to vaccination or infection. By following the developmental fate of antigenspecific B cell populations through analysis of their antibodies we can generate a direct survey of B cell function. In this competitive renewal of our Technology Development Project (TOP) on Human Monoclonal Antibodies we combine two uniquely powerful new technologies recently developed that for the first time allow the efficient analysis of human B cell specificity en mass. One approach pioneered by Dr. Lanzavecchia, provides an analysis of the entire history of B cell specificities by efficiently producing mAbs from the long-term memory B cell compartment. The second technology from the Wilson and Ahmed laboratories scrutinizes current, ongoing plasmablast specificities and generates large numbers of antigenspecific antibodies in a short time that are predominantly specific to the antigens. We will use these tools to address fundamental questions about the human B cell response to yellow fever virus (YFV) and dengue virus. By combining these powerful platforms of generating human mAbs (plasmablasts and memory B cells) we should be able to comprehensively analyze the human B cell response to these viruses and to probe the relationship between memory B cells and antibody secreting plasma cells. Because the antibodies produced are fully human they can yield valuable diagnostics and allow for safer Pharmaceuticals than chimeric or humanized antibodies. Thus, in addition to addressing these fundamental questions, our proposed studies should also result in the development of a large panel of human mAbs against YFV and dengue - two flaviviruses that are of important public health concern and are high priority biodefense pathogens. Three specific aims are proposed: 1) Characterize the primary human B cell response to YFV-17D leading to the generation of ASCs and long-term memory B cells; 2) Analyze the dynamics, variable gene repertoire, and the specificity of ASC and memory cells induced by booster vaccination with YFV-17D. 3) Characterize the human B cell response to acute dengue virus infection of Children.